1. Field of the Invention
The present invention relates to a measuring apparatus, and more particularly, to a measuring apparatus for a camera wherein a signal is projected to an object, and an object distance is measured from an incident position of a reflected signal from the object.
2. Description of the Related Art
In general, there has been known a measuring apparatus as in FIG. 1 using an active autofocus technique of an infrared projection type. Such a measuring apparatus has been used in, for example, a compact camera, VTR, a binocular telescope, and the like. With reference to FIG. 1, light projected from an infrared light emitting diode (IRED) 2 is radiated to an object 6 via a projection lens 4. A reflected light from the object 6 is converged by a receiving light lens 8, and a spot is formed on a position detecting element 10. In this drawing, l is a distance (distance of object) between lens 4 and the object 6, S is a distance (base length) between the lens 4 and lens 8, and f is a focal distance of the lens 8. Moreover, t is a length of the position detecting element 10, and x is a distance from one end of the position detecting element 10 to a position where the reflected light is focused.
According to the above-structured measuring apparatus, if a semiconductor detection element is used as a position detection element, an inverse number 1/l of the distance of the object can be obtained from the following equation (1): ##EQU1##
This semiconductor position detecting element is generally called a position sensitive detector (PSD), and outputs currents I.sub.1 and I.sub.2 depending on a position of the center of gravity of an incident light as shown in FIG. 2A. An image 10.sub.0 of the light signal is formed rectangular. Here, if a total length of PSD 10.sub.1 is t, two current signals I.sub.1 and I.sub.2 can be obtained from the following equations (2) and (3): ##EQU2##
In equations (2) and (3), I.sub.p denotes a total photoelectric current. Then, based on the result of the above equations (2) and (3), x can be obtained from the calculation of ratio shown in the following equation (4): ##EQU3##
There is also known a position detecting element having two photodetectors (silicon photo diode; SPD) 10.sub.2 and 10.sub.3 arranged in parallel. In this case, if a distance from an optical axis of the lens 8 of FIG. 1 to a divisional point is a p, image 10.sub.0 of the light signal is formed rectangular, and a length in t direction is b, the following equations (5) and (6) can be obtained: ##EQU4##
Then, the calculation is performed by use of these equations (5) and (6), the following equation (7) can be established: ##EQU5##
Therefore, even in a case where two SPDs 10.sub.2 and 10.sub.3 are used, x can be obtained by ratio I.sub.1 and that of I.sub.2 if b and p are widely well-known.
Moreover, if each of equations (4) and (7) is combined with equation (1) and the measurement using PSD is performed, the following equation (8) can be established: ##EQU6##
In the case of the measurement using a two-divided SPD, the following equation (9) can be established: ##EQU7##
The above equations (8) and (9) are graphed out as shown in FIGS. 3A and 3B. More specifically, FIG. 3A shows a characteristic view of the inverse number of the measuring distance of the measuring apparatus using the PSD and the output thereof; and FIG. 3B shows a characteristic view of the inverse number of the measuring distance of the measuring apparatus using the two-divided SPD and the output thereof.
In general, in a state that t=about 2 mm and b=about 0.3 mm, an inclination between the inverse number 1/l and the output (I.sub.1 /(I.sub.1 +I.sub.2)) in the SPD measurement is about seven times larger that that in the PSD measurement. In this case, the total length t is not considered. This means that the change of the output is larger than that of the inverse number 1/l, and that higher accuracy can be obtained. On the other hand, in the measurement using the two-divided SPD, unless a part of the spot is always mounted on both SPDs, the measurement cannot be performed. Due to this, as compared with the case of the PSD measurement, the measuring range is extremely small.
In other words, as the change of x of FIG. 1 is increased, the measurable range of l can be increased. However, in the case of b=0.3 mm, the measuring range of the SPD is only 0.3 mm if the value is converted to x. On the other hand, in the measurement of the PSD, there is a measuring range of 2 mm-0.3 mm=1.7 mm, and the measuring range of the PSD measurement is about six times larger than that of the SPD measurement even if the excess of the spot is considered.
In the SPD measurement, if the length is enlarged six times as large as the above value, it is possible to obtain the same measuring range as that of the PSD. However, the above-explained high measuring accuracy cannot be obtained.